[llvm] [SystemZ] Remove incorrect areInlineCompatible hook (PR #152494)

[Nikita Popov via llvm-commits]

nikic wrote:

Probably worth noting that this is not entirely correct for inlining `-vector` into `+vector` where the callee has calls with vector arguments, due to the potential change to the call ABI. However, I don't want to add handling for that as the frontend should usually either reject that or at least warn about it, and trying to handle it is really messy, based on past experience with X86. This PR goes back to the status quo of LLVM 20.

https://github.com/llvm/llvm-project/pull/152494